The present invention relates to a coextruded, biaxially oriented, multi-layer film comprising a base layer composed of high-purity electrical insulation plastic film material having a residual ash content of less than 100 ppm.
This multi-layer film is used as a dielectric in impregnated film capacitors comprising multi-layer films of this type and metal foils. These can be either power capacitors or capacitors which are employed in the field of microelectronics.
The use of plastic films as an electrical insulation material has been known for a long time in the state of the art. The films are used both as a dielectric in capacitors and for cable insulation or as supporting films for self-adhesive insulating tapes.
Films of this type must have certain electrical properties, such as, for example, a high resistance to glow discharges, which are formed particularly at points where very small occlusions of gases are present within the laminated structure, composed alternately of metal foils as electrodes and the dielectric. It is possible for these gases to become ionized by the applied electric field and in this state to be able to attack the dielectric film.
In order to avoid glow discharges, the capacitors are flooded with inert gases or impregnated with impregnating fluids. An example of an inert gas used is SF.sub.6, which can only be ionized with great difficulty. Impregnating fluids are mineral oils or compounds such as trichlorobiphenyl, diethylhexyl phthalate, vinylsilylethane, benzyl neocaprate or isopropylbiphenyl. Impregnating fluids of this type can also be used for the insulation of high-tension cables. German Offenlegungsschrift No. 2,942,298 describes a film which can be impregnated with impregnating fluids of this type.
Polypropylene films are employed as the dielectric in conjunction with thin metal foils as electrodes in capacitors, particularly at very low capacities, for example, 50 to 10,000 pF. In order to achieve an accuracy of 1 to 3% of the indicated capacitance figure, these capacitors are impregnated with the impregnating agents mentioned above. When capacitors of this type are in operation, oxidation and/or corrosion phenomena, which lead to a marked impairment in the electrical properties of the capacitor, occur at the transition between the dielectric and the metal foil. In particular, an increase in the dielectric loss factor takes place as a function of time.
In order to reduce the oxidation phenomena, and hence the dependence of the dielectric loss factor on time, special stabilizers are added to the plastic film. Although these stabilizers in principle lead to a better time-constancy of the dielectric loss factor, at the same time they increase the starting or initial value of the dielectric loss factor in an unacceptable manner.
European Patent Specification No. 0,011,796 discloses for use as an electrically insulating film a biaxially stretched polypropylene film, which is composed of two layers and is used in a metallized form for the production of capacitors. The raw material for this electrically insulating film is high-purity polypropylene having a residual ash content of less than 100 ppm, free from inorganic or organic slip agents and containing no ionic constituents. This known electrically insulating film is prepared in accordance with one of the known processes by melt-coating or coextrusion and subsequent biaxial stretching.